What was found
ENGINE RPM DECREASED CAUSING A LOSS OF POWER IN FLIGHT WHICH FORCED THE PILOT TO AUTO-ROTATE TO ROUGH TERRAIN. THE ACFT CAME TO REST ON ITS SIDE WITH SUBSTANTIAL DAMAGE AND INJURY TO THE PILOT. POST ACCIDENT INVESTIGATION BY OWNER/OPERATOR AND THE OEM TECH REP IDENTIFIED A LOOSE P2 PIPE, AIRLINE IN WHICH THE B-NUT HAD BACKED-OFF FROM THE POST TU231 UNION ON THE ENGINE. THE PIPE WAS LAST DISCONNECTED DURING THE OEM 600 HOUR ENGINE INSPECTION IAW TASK NR 75-29-00-210-801. THE MAINTENANCE TECH RECONNECTED AND TORQUED THE B-NUT AND THE ACFT WAS OPERATED FOR 127.7 HOURS BEFORE THE INCIDENT OCCURRED. ENG MFG HAD ISSUED AN ALERT SL 1807/98-4TH ISSUE DUE TO SIMILAR PROBLEMS OVER THE YEARS WITH LOOSE OR FAILED "AIR SYSTEM PIPES". A RECOMMENDED C/A WOULD BE TO FACILITATE THE APPLICATION OF SAFETY WIRE OR OTHER LOCKING DEVICE TO PREVENT LOSS OF TORQUE ON AIRLINE B-NUTS.